An in-cylinder direct-injection internal combustion engine (hereinafter, also referred to as an in-cylinder direct-injection engine) is an engine of such a type that a fuel injection valve (injector) is arranged in each cylinder, fuel such as gasoline is directly injected into a combustion chamber from the fuel injection valve so that the fuel is mixed with intake air introduced through an intake port into the combustion chamber to form an air-fuel mixture, and the air-fuel mixture is ignited by a spark plug. Demand for the in-cylinder direct-injection engine has rapidly increased, because of its excellence in fuel efficiency, low emission, high power, and the like.
On the other hand, in the in-cylinder direct-injection engine, a time period from fuel injection until the fuel reaches an ignition position is shorter than in a port injection engine injecting fuel into the intake port. Accordingly, atomization of the fuel injected into the cylinder is insufficient at the time of start at an extremely low temperature (for example, −25° C. or lower), which leads to unsatisfactory ignition and poor starting capability.
Methods for solving such problems at the time of start at the extremely low temperature include a method of cutting off ignition, that is, prohibiting ignition for at least one cycle during cranking at the time of start at the extremely low temperature (for example, see Japanese Patent Laying-Open No. 2000-097071) and a method of performing solely fuel injection with ignition by the spark plug being cut off for a prescribed period (ignition cut-off period) at the initial stage of start, and starting ignition along with fuel injection only after the ignition cut-off period has passed (for example, see Japanese Patent Laying-Open No. 11-270387). By carrying out such ignition cut-off, fuel that can contribute to combustion (ignition) can be accumulated in the cylinder.
In the in-cylinder direct-injection engine, however, simply supplying the fuel with ignition being cut off at the time of start at the extremely low temperature leads to occurrence of wetting of the spark plug by fuel (hereinafter, also referred to as plug wetting) and poorer starting capability. Particularly, as an in-cylinder direct-injection engine in which combustion is unsatisfactory due to a shape of the combustion chamber or the like or an in-cylinder direct-injection engine in which friction is large is an engine low in generated torque and hard to start, such an engine tends to experience start failure at the extremely low temperature.
The patent documents mentioned above are silent about such problems of plug wetting at the time of start at the extremely low temperature, and the methods described in these patent documents, that is, the method of prohibiting ignition for at least one cycle during cranking and the method of performing solely fuel injection with ignition being cut off for a prescribed period at the initial stage of start cannot solve these problems.